The present invention relates to an electromagnetically driven shutter device for opening and closing the shutter blades of a camera, and particularly to a driving device for an electromagnetically driven shutter having a current supply arrangement for supplying driving current to an electromagnetic driving source. The driving device is constructed so that the shutter blades stop at a predetermined position in a stable state.
Various types of driving devices for opening and closing shutter blades are used, wherein the device of most simple construction is the electromagnetic driving device. In such device, a moving coil is arranged in the magnetic field of a stator consisting of a permanent magnet or the like. An exciting current is supplied to the coil so as to rotate the coil and thereby drive the shutter blades by making use of the torque developed by the coil.
Further, in a camera having an electromagnetically driven shutter device, since the blades are not mechanically restricted before taking a picture until the device is driven by an exciting current, the shutter blades are then arranged to move freely without any restriction. Therefore, there is a danger that the shutter blades might move due to vibration or shock, and allow a light beam to leak into the camera. Further, unless the blades are always held at the same position before starting to open, a stable and exact exposure time cannot be obtained. Moreover, before the blades have opened, the current supply to the electromagnetic driving source generally is interrupted in order to save power consumption. Accordingly, a braking or holding mechanism is required to prevent free movement of the shutter blades at such time.
In order to prevent free movement of the shutter blades as described above, it is sufficient to provide a mechanism for braking the rotational movement of the moving coil in the electromagnetic driving source at both ends of the path of angular movement through which the shutter driving part must swing, namely, at the start end and at the running termination end. A method, thus, is known wherein at both ends of the path of movement, a permanent magnet is provided so as to attract the blades. Accordingly, more members are needed and such is not economically feasible with regard to manufacturing costs and assembly space.
Further, before the blades have run, the current supply to the electromagnetic driving source is generally interrupted in order to save the power consumption so that the braking holding means is necessary in order to prevent the free movement of the shutter blades.
In order to prevent the free movement of the shutter blades as mentioned above, it is sufficient to provide the both ends of the angle necessary for the shutter driving part to swing between, namely at the start side and at the running termination side, with a mechanism for giving a braking effect to the rotation of the moving coil of the electromagnetic driving source. Hereby, there is a method that at the both end a permanent magnet is provided so as to attract the blades, whereby more members are needed, which is not profitable in the manufacturing cost and the assembling space.